This invention generally relates to processes for manufacturing plastic structures and to containers made from those structures. More specifically, the invention relates to a process for manufacturing laminated plastic structures via an injection molding process and to containers, especially well suited for holding carbonated beverages, foods and chemicals, made from those plastic structures.
Plastic containers are often used to hold carbonated beverages, and in fact, their use for this purpose has steadily grown over the past few years. While many plastics may be used for carbonated beverage containers, polyethylene terephthalate (referred to herein as PET) has achieved a major role in this area because, when manufactured properly, it has many desirable characteristics such as low cost, light weight, durability and rigidity.
Most of the excellent physical properties of PET become evident only when the resin is stretch-molded into a bi-axially oriented condition, and the majority of PET bottles produced for carbonated beverages are manufactured by, first, molding a PET preform via an injection molding process, and second, reforming the preform into the final desired shape. With most PET containers a different machine is employed in each of these two manufacturing steps; however, a significant number of bi-axially oriented PET bottles are also formed in one apparatus having a plurality of positions or stations. Some PET containers not requiring high mechanical properties are produced via conventional injection blow molding and have little or no molecular orientation.
The major cost element in the manufacture of PET bottles is the cost of the PET resin itself. Accordingly, PET bottle suppliers are interested in decreasing the amount of PET resin in the bottles, and one way of doing this is to reduce the thickness of the walls of the bottles. There are lower limits, though, on the thickness of the sidewalls of PET bottles. In particular, PET is gas permeable; and when used to hold a carbonated beverage, the sidewalls of a PET bottle must be thicker than certain minimum sizes if the bottle is to meet industry standards relating to carbon dioxide retention levels. For example, as defined by the industry, the term "shelf life" for a carbonated beverage bottle is the time, in weeks, for the beverage to lose 15 percent of its original carbonation level, when stored at room temperature. The major carbonated soft drink manufacturers in the United States have established a requirement that a PET bottle larger than one liter, when used to package a carbonated soft drink, must have a shelf life of 16 weeks. Bottles less than one liter generally have had to compromise on a shelf life of about ten weeks.
One way to reduce the amount of PET in a bottle and also extend the shelf life thereof, is to use a laminated structure, where the bottle comprises a first layer formed from PET and a second layer formed from a material which has a very low gas permeability (referred to herein as a high gas barrier material).
The use of a laminated bottle structure is advantageous for other reasons as well. For instance, it is highly desirable to employ used plastic material in beverage containers. Such a use, first, would provide a productive outlet for the large number of plastic bottles that are currently simply discarded, and second, would reduce the cost of the materials needed to manufacture new bottles. Government regulations, however, do not allow reprocessed plastic to come into contact with products stored in beverage containers and, instead, require that only unused plastic materials come into contact with those products. One way to meet these regulations while still using reprocessed plastics in a beverage container is to form the container from a laminated structure with a first, outer layer made from the reprocessed material and a second, inner layer made from unused plastic material.
While there are several ways to form a bottle with a laminated structure, it is very desirable to use an injection molding process. Such a process, first, would require relatively few changes to the present processes used by many in the industry and thus could be readily adapted thereby, and second, would have the typical advantages associated with injection molding processes such as dimensional precision and high productivity. Heretofore, however, the art has not been able to develop a practical, efficient and cost-effective injection molding process for manufacturing large numbers of laminated PET bottles.
In part, this is due to the fact that with typical injection molding processes used to make plastic preforms, as a practical matter, it is necessary to make the preform with a slight axial taper. This taper allows the mold to open and permits the preform to be removed therefrom without breaking or tearing the preform. For example, with a prior art method of making a laminated, plastic preform using an injection molding process, a first plastic material is cut, preshaped into a tapered form and placed on a mold core rod. Then, a second, liquid plastic is injected around that rod, and the first and second plastic materials bond together to form the laminated structure. Because of the time and expense needed to preshape the first material into the desired, tapered form and to place it over the core rod of the mold, this general type of method is not believed to be a practical way to manufacture large numbers of plastic preforms. This process also results in the preshaped material being on the inside wall of the formed structure.